1. Field of the Invention
The present invention relates to producing specialty materials and chemical intermediates from bio-renewable feedstocks such as animal fats, plant oils, algal oils, bio-derived greases, and tall oil fatty acid (hereafter referred to as biological feedstocks, or alternatively, fatty acids and/or glycerides depending upon the composition of the feedstock). Specifically, the present invention relates to predominantly even carbon number paraffin compositions in the C12-C24 range, and the catalytic hydrogenation/hydrogenolysis method used for its manufacture.
Paraffins in the C12-C24 range have useful applications as phase change material (PCM). The paraffins undergo solid-liquid phase transition in the about −9° C. (15° F.) to about 50° C. (120° F.). Heat is absorbed as the PCM paraffin melts and heat is released later when the PCM freezes. Fabricated systems that use PCM's as such are referred to as passive thermal storage devices. Due to relatively high latent heats of solid-liquid phase transition (referred to simply as latent heats hereafter), as well as compatibility with common material of construction and high stability, paraffins are considered particularly well-suited for PCM applications. Wall boards of a house impregnated with a PCM are an example of a passive thermal storage device. During a hot day, the PCM will absorb heat as it melts. Since there is no temperature change during phase transition, the surface in contact with the thermal storage device stays at constant temperature until all PCM therein has melted. The heat that would have made the house hot has thus been stored in the molten PCM. At night, as the temperatures get cooler, the molten PCM freezes and releases the heat thus preventing the home from getting cold. The melting-freezing cycles moderate the temperature of the space enclosed within the passive thermal storage device despite extreme night-day temperature swings outside. In general, PCMs are an effective way of storing thermal energy (e.g. solar, off-peak electricity, industrial waste heat), and reducing energy demand (e.g. for heating and air-conditioning).
The thermal storage capacity of the PCM is dictated by its latent heat. The higher the latent heat, the higher the thermal storage capacity of the PCM, and the smaller the required thermal storage device size/cost.
Table 1 provides the latent heats and melting points of paraffins. As observed therein, the latent heat for even carbon number paraffins is higher than the latent heat for odd carbon number paraffins of similar transition temperature. For example, n-heptadecane (carbon number 17) and n-octadecane (carbon number 18) melt in the 22-28° C. range. Whereas the latent heat of n-heptadecane is 215 kJ/kg and the latent heat for n-octadecane is 245 kJ/kg or 14% higher. In general, the even carbon number paraffin heats of fusion in the C14-C24 range are 10-16% higher than odd carbon number paraffins.
TABLE 1Latent Heats and Solid-Liquid Transition Temperatures of SelectedParaffinsMelting PointNameCarbon Number(° C.)Latent Heat (kJ/kg)n-Tetradecane144.5-5.6231n-Pentadecane1510207n-Hexadecane1618.2238n-Heptadecane1722215n-Octadecane1828.2245n-Nonadecane1931.9222n-Eicosane2037247n-Heneicosane2141215n-Docosane2244249n-Tricosane2347234n-Tetracosane2451255
In addition to PCM applications, even carbon number C12-C24 paraffins are also used as chemical intermediates for linear alkyl benzene (C12, C14) and alkyenyl succinate (C16, C18), as well as lubricant/wax additives.
2. Brief Description of the Related Art
The commercially practiced synthesis of even carbon number n-paraffins involves ethylene oligomerization. Depending on the catalyst and reactor operating conditions, this process produces a distribution of linear alpha olefins in the C4 to C20+ range. Linear alpha olefins in the C4-C8 range are the main products of this process and are separated. These olefins are in high demand, mainly as comonomers for film-grade polyethylene. The C10+ even carbon number olefins are sold as intermediates for specialty chemicals, or hydrogenated to produce even carbon number n-paraffins.
This ethylene oligomerization process for producing even carbon number paraffins is highly dependent on crude oil and natural gas prices. Furthermore, n-paraffins have to be sold at a premium to the olefins to justify the added cost of hydrogenating. These factors make the price and availability of n-paraffins thus produced highly variable.
Another method of producing n-paraffins is Fischer-Tropsch synthesis. The liquid product of this reaction is a broad distribution of even and odd carbon number paraffins, from C5 to C50+.
Naturally occurring fatty acids and esters may be hydrotreated to produce a hydrocarbon composition including even and odd carbon number paraffins as reported in prior art, namely: Wong, et. al. “Technical and Economic Aspects of Manufacturing Cetane-Enhanced Diesel Fuel from Canola Oil”; Bio-Oils Symposium, Saskatoon, Saskatchewan, Canada; March 1994. FIG. 2 of Wong et. al. includes typical gas chromatography/mass spectrometry (GC/MS) trace of hydrotreated canola oil wherein the relative heights of even and odd carbon number paraffins are similar, indicating presence of comparable concentrations of each. The prior art method for converting triglycerides and fatty acids to paraffins employs a fixed-bed catalytic reactor, packed with commercially available hydrotreating catalysts. These catalysts are cylindrical or three-fluted extrudates of alumina with nickel molybdenum or cobalt molybdenum sulfided metal activity. The typical equivalent diameter of these catalysts is from about 1.5 mm to about 2.0 mm.
The equivalent diameter is used to characterize non-spherical particles by size. Equivalent particle diameter of a non-spherical particle is defined as the diameter of a sphere having the same volume as the non-spherical particle. For a cylindrical catalyst of diameter D and length L, the equivalent particle diameter Dp is expressed as Dp=6(4/L+4/D)−1. For a three-fluted extrudate, the equivalent particle diameter expression is) Dp=6[2/L+5π/(D(sin(60°)+1.25π)]−1.
The fatty acid/glyceride feed is hydrogenated and deoxygenated in the fixed bed reactor packed with commercial hydrotreating catalysts. As illustrated by Equations 1 and 2 for the example of triolein (oleic acid triglyceride), the deoxygenation is achieved by oxygen hydrogenolysis, decarboxylation (removal of CO2), and decarbonylation (loss of CO).

In both reactions, the glycerol backbone is converted to propane and double bonds are saturated. Since one carbon is removed from the fatty acids during decarboxylation and decarbonylation reactions (as illustrated in Equation 2), odd carbon number paraffins are formed from even carbon number naturally occurring fatty acids.
To this end, there is a need for even carbon number paraffin compositions and a selective process for producing even carbon number paraffins. In particular, the present invention is a process for converting biological feedstocks into even carbon number paraffin compositions.